A variety of industrial processes are known for conversion of low boiling carbon-containing compounds to higher value products. One such process converts methanol to gasoline (MTG) using ZSM-5 zeolite catalysts. In the MTG process, methanol is first dehydrated to dimethyl ether. The methanol and/or dimethyl ether are then converted in a series of reactions that results in formation of a hydrocarbon mixture that comprises aromatic, paraffinic, and olefinic compounds. This mixture may be separated into a liquefied petroleum gas (LPG) fraction and a high-quality gasoline fraction comprising aromatics, paraffins, and olefins. The typical MTG hydrocarbon product consists of about 10-20% olefins and about 40-50% paraffins and 40-50% aromatics, e.g., xylenes, benzene, toluene, etc.
U.S. Pat. Nos. 6,423,879 and 6,504,072 disclose processes for the selective production of para-xylene which comprises reacting toluene with methanol in the presence of a catalyst comprising a porous crystalline material having a Diffusion Parameter for 2,2 dimethylbutane of about 0.1-15 sec−1 when measured at a temperature of 120° C. and a 2,2 dimethylbutane pressure of 60 torr (8 kPa). The porous crystalline material is preferably a medium-pore zeolite, particularly ZSM-5, which has been severely steamed at a temperature of ≥950° C. and which has been combined with about 0.05 to about 20 wt % of at least one oxide modifier, preferably an oxide of phosphorus, to control reduction of the micropore volume of the material during the steaming step. The porous crystalline material is normally combined with a binder or matrix material, preferably silica or a kaolin clay.
U.S. Pat. No. 7,304,194 discloses a process for the hydrothermal treatment of a phosphorus-modified ZSM-5 catalyst. In the process, ZSM-5 having a silica/alumina mole ratio of ≥about 250 and a phosphorus content of from ≥about 0.08 g P/g zeolite to about 0.15 g P/g zeolite is calcined at a temperature of ≥300° C. and then contacted with steam at a temperature of from about 150° C. to about 250° C. The steamed, phosphorus modified zeolite is said to exhibit improved para-selectivity and methanol selectivity when used as a catalyst in toluene methylation reactions. The steamed, phosphorus modified zeolite may be used as a catalyst in unbound form or in combination with a binder material, such as alumina, clay, and silica.
In addition, U.S. Pat. No. 7,285,511 discloses a process of modifying a zeolite catalyst to increase its para-xylene selectivity in toluene methylation reactions, wherein the method comprises forming a slurry consisting essentially of a binder-free ZSM-5-type zeolite having a SiO2/Al2O3 mole ratio of from about 250 to about 1000 and an aqueous solution of a phosphorus-containing compound; and removing water from the slurry to provide a non-steamed, phosphorus treated ZSM-5 zeolite having a phosphorus content of from 0.04 g P/g zeolite or more and a pore volume of from 0.2 ml/g or less. The resultant phosphorus treated ZSM-5 can be used as a toluene methylation catalyst either in unbound form or may be composited with a binder, such as alumina, clay, or silica.
Phosphorus modification is a known method of improving the performance of zeolite catalysts for a variety of chemical processes including, for example, the conversion of methanol to hydrocarbons and the methylation of toluene to produce xylenes. For example, U.S. Pat. Nos. 4,590,321 and 4,665,251 disclose a process for producing aromatic hydrocarbons by contacting one or more non-aromatic compounds, such as propane, propylene, or methanol, with a catalyst containing a zeolite, such as ZSM-5, together with a binder or matrix material resistant to the temperature and other conditions employed in the process. The zeolite is modified with phosphorus oxide by impregnating the zeolite with a source of phosphate ions, such as an aqueous solution of an ammonium phosphate, followed by calcination. The phosphorus oxide modification is said to render the zeolite more active and/or benzene selective in the aromatization reaction.
Nevertheless, there is still a need for catalyst compositions having improved selectivity and/or resistance to deactivation and/or persistence in the selectivity of aromatic selectivity.